Chris,
At the end of the day, speed will be determined by HP at the flywheel, less drivetrain loss's. So if you have the same motor in a rig with a 9.5" rear diff, vs a 10.5 vs an 11.5. The 11.5 diff will use up more of the motors HP, so it will g slower up a given grade than the one with the 9.5" rear. If the brake drums, calipers etc are smaller on one, that too will get more HP to the ground, so it should be a faster rig up the hill. Put an Allison behind the GM 8.1, and it will be a slower rig up a hill vs the 4l80E trannys, which will be slower than the NV manual trannies, as the allison uses close to 50 hp to run, vs 30-35 for the 4l80 trans vs 20 for the NV.
Lower axel gears will not make up for these loss's. Now if you can not pull say a 20% grade with 4.10's, everything else being equal, a rig with 4.88's may go up the hill, as torque is multipliable. Or the rig with the allison may go up a steeper grade than the 4l80 due to its lower first gear ratio. as will an nv go upa 15% greater hill than an allison and some 20% greater than a 4l80!
Gears help at times, others it is how many HP hit the ground at the wheels. For the OP, 4.10s may be the better option than 4.88's. I find for how I drive, if I can get everything else correct, 4.10s with a typical .75 od and 31" tires is best. 4.33's would work well if I have a typical. .65 dod in the trans, potentially a 4.56 depending upon the motor, rpm it wants to cruise at etc. That is with my typical 60 mph on the interstate driving, be it empty or loaded.
This is like a 28" tired rig with 3.42 gears, will turn the same rpm, shift at the same places as a rig with 30" tires and 3.73 gears, as is a rig with 32" tires and 4.10 gears, If everything else is the same, these rigs will perform about the same.
Marty
